f2fs: add a help func F2FS_STAT() to get the f2fs_stat_info

Add a help func F2FS_STAT() to get the f2fs_stat_info.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>